Optical waveguides can be formed in polymers by using a core polymer and a cladding polymer with the core polymer refractive index slightly higher than that of the cladding polymer in the near infrared region of the third optical telecommunication wavelength window (around 1550 nm). In order to form useful optical waveguide devices such as integrated splitters, couplers, arrayed waveguide gratings, and optical waveguide amplifiers, it is essential to have stable and low loss optical waveguides. The optical loss, or attenuation of an optical waveguide, originates primarily from two sources: 1) optical absorption by the core and cladding material and 2) optical signal scattering from the waveguide.
A general approach to making polymer optical waveguides is to dispose an undercladding polymer film layer on a silicon substrate and then a polymer core film layer on top of the undercladding layer. The polymer core layer film subsequently undergoes lithography and etching processes from which a rectangular cross-section channel is formed An overcladding polymer film layer is then disposed on top of the waveguide core and the exposed undercladding film layer.
It has been found that, during the processes of forming the undercladding, core and overcladding layers, such as spin coating and subsequent drying of solvents, temperature variations usually occur throughout the polymer waveguide layers. Such temperature variations cause polymer shrinkage or expansion in accordance with thermal expansion coefficients (CTE) of the polymer materials, which typically run between approximately 50 to 300 parts per million (ppm) per degree Celsius, depending on the particular polymer. Generally simultaneously, the waveguide substrate undergoes similar shrinkage or expansion as the temperature changes. However, in contrast to the CTE for polymers, the CTE for silicon is approximately 4.2 ppm per degree Celsius. The mismatch of CTE between the silicon substrate and the polymer waveguide claddings and core can cause polymer film cracking and stress build-up in the polymer layers. These effects will increase the polymer waveguide attenuation, preventing practical waveguide device application of polymer waveguides. This tendency can be further quantified through the following expression:σf=Ef(CTEf−CTEs)(Tproc−Tamb)  Equation 1where:                σf is the stress in the film;        Ef is the elastic modulus of the film;        CTEf is the CTE of the polymer film;        CTEs is the CTE of the substrate;        Tproc is the processing temperature; and        Tamb is the ambient temperature.        
It is believed that polymers have been used as a substrate as well as the waveguide disposed on the substrate. Keil et al. have disclosed fluoroacrylate-type polymers such as pentafluorostyrene, trifluoroethylmethacrylate, and glycidylmethacrylate disposed on a polymer substrate. However, these fluoroacrylate-type polymers contain numerous CH bonds. Polymers with CH bonds typically have high absorption in the infrared region where the optical communication signals reside, at approximately 1.5 μm. This absorption causes optical communication signal loss. To alleviate the signal loss problem, CF bonds are used to substitute the CH bonds in the polymer. Perfluorinated polymers have no CH bonds, resulting in extremely low absorption loss around the 1.5 μm infrared communication wavelength.
It is desirable to have a low loss optical waveguide in which the coefficient of thermal expansion of both the substrate and the polymer layers disposed oil the substrate are such that the polymer layers do not crack or develop high stress on the substrate.